The present invention relates to apparatus for cutting paper sheets or the like, especially for severing running webs of paper to form elongated strips which are thereupon subdivided into smaller sheets of the type used in steno pads, note books or the like. More particularly, the invention relates to improvements in means for moving a rotary knife of such cutting apparatus with reference to a rotary counterknife.
It is already known to sever a running paper web in an apparatus wherein a driven counterknife cooperates with a disk-shaped knife which is freely rotatable on its shaft. The apparatus comprises a first motor which moves the knife sideways (i.e., substantially at right angles to its axis) toward and away from the counterknife, and a second motor which thereupon moves the knife axially into engagement with the counterknife so that the latter transmits torque to the rotary knife while the two implements cooperate to cut the running web. In such apparatus, sidewise movement of the rotary knife must be terminated prior to start of the axial movement in order to insure that the cutting edge of the rotary knife does not strike against the counterknife. As a rule, to extent to which the marginal portions of the two implements overlap when the rotary knife has completed its sidewise movement toward the counterknife is in the range of one millimeter. Such minute overlap suffices to result in damage to the rotary knife and/or counterknife if the rotary knife is permitted to strike against the counterknife during sidewise movement toward the counterknife.
A proposal to avoid undesirable contact between the two implements during sidewise movement of the rotary knife is disclosed in German Auslegeschrift No. 1,156,635. Each of the two motors is a pneumatic motor and both motors receive pressurized fluid from a common source. The pistons of both motors are biased by springs, and the bias of the springs is selected in such a way that, when the source is free to admit pressurized fluid to both motors, the piston of the first motor is displaced prior to displacement of the piston which forms part of the second motor. This is achieved by appropriate selection of the bias of springs for the respective pistons.
A drawback of the just described proposal is that the piston of the second motor is displaced by fluid which is maintained at an elevated pressure. Therefore, the apparatus must be provided with a stop to arrest the axial movement of the rotary knife in a direction toward engagement with the counterknife. The rotary knife is free to wobble on its shaft and is biased by a dished spring which determines the pressure between the two implements in actual use of the apparatus.
In accordance with a further prior proposal, the piston of the second motor is not biased by a spring.